frontend: run tests in ci in single thread

CI sometimes still hangs, trying to run without threads

- vitest-dev/vitest#2008

scripts/ci.sh

@@ -27,7 +27,7 @@ golangci-lint run
 
 npm --prefix=frontends/web install # needed to install dev dependencies.
 make weblint
-npm --prefix=frontends/web test -- --no-color --no-watch
+npm --prefix=frontends/web test -- --no-color --no-watch --no-threads
 # check that the i18n files are formatted correctly (avoids noisy diff when
 # pulling from locize)
 if ! locize format frontends/web/src/locales --format json --dry true ; then